The present invention relates to a phase shift circuit suitably used for a device required to generate signals having a specific phase difference and substantially the same amplitude.
A phase shift circuit is designed to shift the phase of an input signal on the basis of the phase shift characteristics of the circuit and output the resultant signal, and has various applications including an image rejection mixer circuit.
Phase shifters used in an image rejection mixer should be designed without phase error and amplitude error to get highest image rejection rate. However, the actual phase and amplitude of a phase shift output signal considerably differ from target values owing to an input signal frequency deviation from the center frequency in the phase shift circuit or variations in the constants of elements constituting the circuit from design values.
For this reason, the image signal is not satisfactorily rejected owing to the phase and/or amplitude error of a 90.degree. phase shifter.
Under the circumstances, the amplitude difference between phase shift output signals is detected and corrected, or the phase difference between the signals is detected to control the phase shift characteristics of the phase shifting device.
For example, Japanese Patent Laid-Open No. 3-60206 (FIG. 11, in particular) and Japanese Patent Laid-Open No. 8-130416 (FIG. 5, in particular) disclose the former method. Japanese Patent Laid-Open No. 3-60206 (FIG. 10, in particular) discloses the latter method.
According to the former method, however, if an amplitude error is corrected by feed forward control, a power detection circuit itself must be corrected to perfectly correct the amplitude error. This poses a problem in term of design.
In addition, when an amplitude error is corrected by feedback control, the amplitude error cannot be perfectly corrected, thus producing a steady error.
More specifically, although an amplitude difference approaches 0 by loop control, the amplitude difference changes to the positive side after it passes 0. When control is performed to return the amplitude difference on the positive side to 0 again, the amplitude difference changes to the negative side. By repeating this operation, the amplitude difference finally stays within a certain error range (steady error).
According to the latter method, since a phase error is corrected by feedback control, the phase error cannot be perfectly corrected, and a steady error is produced, as in the case of feedback control for an amplitude error.